The present invention relates to an improved process for recovering elemental phosphorus from sludge, and more particularly from the sludge formed during the production of elemental phosphorus by the smelting of phosphate rock.
The basic method for producing elemental phosphorus is accomplished by the reduction of phosphate rock with coke or other carbonaceous reducing agents in the presence of silica. This is referred to as the furnace "charge" or "burden." The phosphorus production is generally carried out in an electric furnace at a reaction temperature of about 1,400.degree.-1,500.degree. C.
The mechanism of the reduction of phosphate rock to elemental phosphorus is quite complex and the exact path of the reaction sequence has not been conclusively defined. The overall reaction is generally represented by the following simplified equation: EQU Ca.sub.3 (PO.sub.4).sub.2 +3SiO.sub.2 +5C.fwdarw.3CaSiO.sub.3 +5CO+P.sub.2
During the course of the reaction, the phosphorus produced vaporizes, rises, and is cooled, condensed, and collected under water. The phosphorus vapor is generally accompanied by carbon monoxide and appreciable quantities of entrained dust comprising phosphate burden, K.sub.2 O, fluorine, lime, slag particles and the like. The dust-vapor mixture can be passed through a Cottrell electrostatic precipitator where most of the dust is removed prior to cooling the phosphorus vapor. The CO gas can be recovered for use as fuel or properly disposed of in accordance with pollution requirements. Solid furnace residue comprising calcium silicate is drawn off from the bottom of the furnace as a molten liquid. Iron phosphide or "ferrophosphorus" formed from the iron impurities present in the phosphate ore is also drawn off as a melt from the bottom of the furnace.
The condenser drains into a sump wherein the phosphorus product is collected. Three separate layers generally form in the condenser sump.
A layer of relatively high grade phosphorus is obtained at the bottom. The intermediate layer is a mixture referred to as "sludge," which consists of phosphorus droplets or globules, solid impurities, and water. Above the sludge layer is a water layer. The boundary between the sludge layer and water layer is not clearly defined.
The amount of phosphorus sludge (P.sub.4 -sludge) produced will vary, depending upon factors such as the initial composition of phosphate rock charged, the operating conditions and design of the furnace. Phosphorus sludge can contain from about 5% to about 90% by weight elemental phosphorus. The phosphorus sludge produced can vary from about 10 to about 60 weight percent or more, of the furnace output of elemental phosphorus.
The elemental phosphorus can be recovered from the sludge by roasting. This operation is becoming more expensive due to rising energy costs. It is also hazardous due to pressures generated inside the roaster. In addition, roasting leads to high P.sub.2 O.sub.5 emissions, a pollution problem. Other methods for treating the sludge include burning it and making low grade phosphoric acid.
The sludge, as the term is used in the art and herein, appears to be a poorly defined emulsion containing solid impurities, water and phosphorus in widely varying proportions and having a density between the density of phosphorus and the density of water. The sludge can have the characteristics of a "phosphorus in water" type emulsion, that is, the phosphorus being the discontinuous phase and the water being the continuous phase, or a "water in phosphorus" type emulsion, that is, the water being the discontinuous phase and the phosphorus being the continuous phase. Microscopic examination of the phosphorus in water type sludge shows that the phosphorus is present in small globular particles which will not coalesce. The size of the particles is generally in the range of micron to millimeter, and larger dimensions.
Various methods for recovering elemental phosphorus from sludge have been suggested in the prior art. Among these are physical methods for separating the phosphorus, such as filtration, distillation, stirring and settling, vibration, centrifuging, briquetting and the like. There have also been chemical innovations for recovering phosphorus from sludge in the patent literature. For example, U.S. Pat. No. 3,084,029 discloses that when small amounts of dispersing agents are introduced into the sludge and the pH of the sludge adjusted to about 6 the viscosity and stickiness is reduced. U.S. Pat. No. 3,104,952 admixes the sludge with phosphoric acid and steam distills the mixture in a non-oxidizing atmosphere.
U.S. Pat. No. 3,436,184 uses an oxidizing agent to decrease the phosphorus content of the sludge. U.S. Pat. No. 3,442,621 treats the sludge with chromic acid and coalesces the phosphorus in the sludge in the presence of an acidulating agent in order to decrease the phosphorus content of the sludge. U.S. Pat. No. 3,615,218 recovers the phosphorus values from the sludge by admixing the sludge with carbon disulfide.
The present invention has achieved the recovery of elemental phosphorus from sludge by subjecting the sludge to electrolysis, thereby enabling the facile separation and recovery of elemental phosphorus.